Electronic Structure of Ti3+–Ethylene Complexes in Microporous Aluminophosphate Materials. A Combined EPR and DFT Study Elucidating the Role of SOMO Orbitals in Metal–Olefin π Complexes

The interaction of tetrahedrally coordinated Ti3+ ions generated in the framework of TiAlPO-5 microporous materials with 12,13C2H4 leads to the formation of side-on η2 {Ti3+-C2H4} complexes with a unique 5-fold coordination of titanium, supported by four oxygen donor ligands of the framework. The detailed electronic and magnetic structure of this adduct is obtained by the combination of advanced EPR techniques (HYSCORE and SMART-HYSCORE) in conjunction with periodic and cluster model DFT calculations. The binding of C2H4 results from the σ overlap of low lying C2H4 filled π orbitals with the 3dz 2 empty orbital of titanium, enhanced by a small contribution due the π overlap between the semioccupied 3dyz orbital of titanium and the empty π∗ orbital of ethylene. The spin density repartition over the ethylene molecule, obtained experimentally, allows probing directly the entity of the metal-to-substrate π∗-back-donation, highlighting an asymmetry in the spin density delocalization. This interesting feature is supported by parallel theoretical calculations, which cast the role of the oxygen donor ligands in driving this bonding asymmetry. As a consequence, the interesting structural feature of potential and actual inequality in the electronic spin states (α,β) on the two ethylene carbon atoms of the π coordinated ethylene molecule is produced. The underlying electronic effects associated with the π coordination of ethylene to an early transition metal in paramagnetic state are thus revealed with an unprecedented accuracy for the first time.